Heat Transfer Fluids and Thermal Oils Information

CoolantsHeat transfer fluids and thermal oils are used to carry thermal energy in process heating, metal working, and machine cooling applications. There are several types of products. Circulating coolants, chiller fluids, anti-freezes, and refrigerants are used to transfer heat or provide cooling within machinery, process equipment, or combustion engines. Hot oils, heater oils, and other thermal fluids are used to provide or transfer heat to a region near machinery or process equipment. Typically, products such as refrigerants, hot oils, and anti-freezes provide indirect heating or cooling and do not come into contact with the machinery or equipment. By contrast, metal working fluids and quenchants contact the workpiece or component directly. Most heat transfer fluids and thermal oils are used in circulating process heaters, chillers, or coolers between a heat source and heat sink. Primary refrigerants provide cooling via phase changes or a Rankine cycle that consists of evaporative cooling and heat transfer. Secondary refrigerants include both circulating coolants and anti-freezes.


Heat transfer fluids and thermal oils vary in terms of chemical composition. Synthetic products include ester and diester compounds, polyglycol and water-glycol fluids, and silicone-based greases and oils. They are formulated from alkaline organic and inorganic compounds and used in diluted form with concentrations ranging from 3% to 10%. Non-synthetic products consist of petroleum or mineral oils and may include water. Synthetic products are more expensive than non-synthetic fluids, but provide better fire resistance and cooling performance. The cost and heat transfer performance of semi-synthetic fluids falls between those of synthetic and soluble oil fluids. Chlorofluorocarbons (CFCs) are nonflammable chemicals that contain carbon, chlorine, and fluorine. Halogenated fluorocarbon (HFC), halogenated chlorofluorocarbon (HCFC), and perfluorocarbon (PFC) fluids are included in this group.


Heat transfer fluids and thermal oils vary in terms of:

  • kinematic viscosity
  • operating temperature
  • pour point
  • boiling point
  • flash point

Kinematic viscosity is the time required for a fixed amount of fluid or oil to flow through a capillary tube under the force of gravity. Units of measure include stoke, centistoke (1/100 of stoke), and Saybolt universal seconds (SUS). Pour point, the lowest point at which a fluid flows, is generally 15° F to 20° F below the system’s lowest end-use temperature to prevent pump damage through cavitation. Flash point is the lowest temperature at which a liquid produces sufficient vapors to form an ignitable mixture in the air near the surface. Fire point and autogenous ignition are important considerations when selecting fire resistant fluids. Fire point is the lowest temperature at which a liquid can produce sufficient vapors to form a mixture in the air that continuously supports combustion after ignition. Autogenous ignition (AIT) is the temperature at which ignition occurs spontaneously.  


Heat transfer fluids and thermal oils are used in a variety of applications and industries. Some products are used in aerospace, automotive, marine, or military applications. Others are used with combustion engines, processing equipment, compressors, piston pumps, gears, and final drives. Specialty heat transfer fluids and thermal oils can inhibit or prevent oxidation, corrosion, or microbial growth. Biodegradable products are designed to break down into harmless chemicals when released into the environment. Non-foaming characteristics are achieved through the use of additives that break out entrained air. Heat transfer fluids and thermal oils that are rated food, beverage, and pharmaceutical applications are also available.

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